Microphone for the deaf



Patented Jan. 23, 1934 UNITED STATES PATENT OFFIC signal to Siemens & Halske Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application August 15, 1930, Serial No. 475,576,, and in Germany August 17, 1929 2 Claims. (or 179-427) d Tile invention relates to a microphone for the In portable electrical apparatuses for the deaf, there are a series of requirements which in 5 theirtotality can only be satisfied by a compromise.

The object of the invention is to contrive an apparatus in which the various requirements are satisfied more completely than has hitherto been possible. The requirement of as large a sound as possible without using a separate electrical amplification makes it? necessary to utilize in every case mechanical resonance. The natural vibration of the microphone must therefore be situated in the range of the important or characteristic speech frequency band, which has distortions as a result.. By characteristic speech frequency band I mean such portion of the complete frequency band as is sufficient to give intelligible speech reproduction, even though such reproduction may not be as accurate as if the entire speech frequency band were used. To obtain greater loudness, generally at least two microphones are used. These microphones are'normally tuned approximately alike, but the resonances naturally do not coincide. In the co-operation of such microphones, difficulties arise as they are electrically independent of each other. The current fluctuations of the two microphones undergo considerable phase displacements in passing through the two resonance fields. As regards the frequency phase, this is, with the usual apparatuses, not very favourable in consequence of thevery strong articulated resonance. This is, however, necessary as the carbon contacts have a comparatively high threshold of sensation and only the storage of energy caused by resonance, even with small exciting loudnesses permits .reliable excitation of the contacts above their level. In principle, a covering of a wide resonance range is possible by coupled systems. Thus, for example, a microphone has been coupled with a suitable tuned air space. and a broadening of the resonance curve-has been obtained with this in known manner. Such apparatuses-are, however, necessarily large and particularly so in respect to the thickness, and this in apparatuses for the deaf which, as far as possible are to be worn under the clothing, is very inconvenient. With the use of several microphones, the utilization of the space is very bad since the microphone casings are usually round and thus there is a good deal of waste space. Furthermore, the walls must be made double as two lateral edges but is free at the ends.

the prepared casings must usually be surrounded by a casing of rectangular shape. It is also not possible to make the microphone cases very flat, although a very flat construction of thecontact bodies in itself is possible. This depends on the fact that the tuning of the diaphragms is greatly influenced by the air spaceunderneath. Very flat microphone casings are, for this reason, insensitive. The utilization of the space is consequently bad in both directions. On the one hand, the free surface of the sound receiving casing is much greater than the effective surface of the two microphone diaphragms, and, furthermore, the thickness of the sound receiver is necessarily over its whole extent at 7 least as great as the, separate microphones necessitate for reasons of snsitiveness.

The above-mentioned difilc'ulties of the strong- 1y enunciated natural vibration and the bad utilization of space, are avoided according to the invention by any desired number of microphones being used electrically with respect to the contact arrangements, but acoustically only one receiving diaphragm.-

This pick-up diaphragm is preferably of rec- 0 tangular shape and forms the one limiting wall of a flat rectangular box in which the carbon contact bodies are suitably arranged. By these means, it is possible to obtain a very flat microphone, as for the reaction of the air space on 5 the diaphragm the volume is of the greatest importance and consequently the utilization of the hitherto useless spaces the whole of the rectangular surface being available, permits a smaller-thickness of construction.

The resonance curve of such a vibration formation can be very favourably' formed for the present purpose, as it can be suited within wide limits to the requirements. In order to make the decisive conditions understandable, we will take the case of a band shaped oscillating surface, that is, a rectangular plate which is clamped at its Such a band has obviously a system of natural vibrations, namely, a strongly enunciated basic sound, and corresponding over-tones, agreeing essentially with the basic sound of the natural vibration of a cross strip element. The length of the band is for the most part without influence. This applies also approximately for the case when the two short sides are also clamped, when the band is very long. If the band becomes shorter and shorter, then a great alteration takes place in the resonance curve. Thereis now added a second system of natural tones which is determined for most part by the natural vibration of a longitudinal strip element. Ihe two systems of natural vibrations are, however, intimately coupled, and infiuence each other in the manner known with coupled systems. It is clear that with suitable dimensioning, a comparatively wide resonance curve is obtained with two maxima.

From what has been stated above, it is clear that the arrangement on account of the difierent variables, can be largely suited to requirements. A special advantage of the improved arrangement is the circumstance that with the fixing of the diaphragm, it is not necessary to arrange any contact make therewith. A certain amount of difiiculty has arisen with ordinary microphones owing to the fact that the current through the diaphragm must be carried through the casing, and that contact dificulties are created owing to aluminium cases being used on account of the weight. The improved arrangement permits any desired fixing of the circumference of the diaphragm without contact make. This circumstance permits a special space economizing and reliable fixing of the diaphragm, for example, by varnish. This method of fixing has been proved particularly good with the new microphone, as the method provides a means of fixing free of cracking noises and ensures an intimate mechanical connection with the casing, so that the mass of the casing body is fully utilized and no vibration energy is lost by damping in the clamping. A further advantage of the arrangement consists in the possibility of using any desired material for the diaphragm. The conduction of the current is best carried out in the case of a non-conductor by a thin strip of carbon which is stuck on the diaphragm at a suitable position and covers the contact body at a suitable distance. The current runs in this case from one contact body through the movable carbon contacts to the diaphragm through the carbon strip, over the movable contacts to the second body. The conditions are particularly simple with the use of a homogeneous carbon plate as the diaphragm.

It may, furthermore, be advisable in order to obtain a wide frequency range and good accommodation to the supplementary apparatuses, to fill the carbon receptacle with grains of different properties. The different properties may be produced-by different kinds of grains or by different sizes with the same kind of grain.

In the accompanying drawing an example of carrying out the invention is shown.

Fig. 1 shows an elevation, Fig. 2 a section in the direction AB of Fig. 1, Fig. 3 is a section on the line C-D of Fig. 1.

The casing 1 of rectangular shape with suitably rounded comers contains inside, the two carbon blocks 2, 3, in the depressions 4, 5 of which are carried the carbon grains 6. On the up-turned edge 7 of the casing 1, is glued the carbon diaphragm 8 in such manner that it bears almost entirely on its edge. The casing 1 is finally closed with the cover 9 which is made thicker in the centre than at the edges, in order to attain the shape of a bulged pocket-book.

Over the parts of the carbon diaphragm 8, which rests above the carbon blocks 2, 3, the cover 9 is provided with a series of cylindrical holes in order to conduct the sound coming from outside on to the carbon diaphragm. In this way, the usual arrangement of two superposed covers is avoided, which in the use of separated microphone cases could not be obviated, and which take up a great deal of room since for acoustical reasons the outer cover had to be arranged at a certain distance from the perforated cover of the microphone casing. Thus, not only is the rectangular base surface of the case smaller, but the thickness is considerably reduced.

Two contact pins 10 are arranged on the narrow side of the casing,- and. these are electrically connected with the carbon blocks 2, 3. On these pins are pushed the corresponding plug sockets provided on the connecting lines. Furthermore, a clip 11 in the form of a strip ofunetal provided with a globular end 12 is fixed on the back of the casing, so that the case may if necessary be hooked on to the clothing.

I claim:

' 1. A transmitter of the class described consisting of a plurality of separate microphones preferably connected in series, a rectangular diaphragm having longitudinal and transverse resonances both substantially within the range of the characteristic speech frequency band to be transmitted, a case containing the diaphragm and the microphones, said diaphragm being common to the microphones and adapted to vibrate bodily as a whole to develop at least two different resonance frequencies of which the one, a longitudinal resonance, is determined chiefly by the length of the diaphragm and the other, 'a transverse resonance, by the width thereof.

2. A transmitter for the deaf, comprising a plurality of separate microphones, a rectangular diaphragm common to said microphones and having longitudinal and transverse resonances' 

